1. Field of the Invention
This invention relates to color printing, more particularly to producing high quality printed images using light black ink.
2. Background of the Invention
Color printing typically uses cyan, magenta, yellow and black (CMYK) inks or toners to produce full-color printed images. The term ink is defined to include toners, inks and any other color delivering solution in a printer system. Printed images are formed from dots of these colors in a combination determined by the desired color to be printed. However, at lower resolution such as 600 dots per inch (dpi), the black dots become very noticeable. The resulting images appear grainy, especially when the black ink used inkjet pigment black ink. This particular ink is a different material than the dye-based CMY inks used.
Several different solutions to this problem have been proposed. Two common approaches are to print using CMY with no K or to modulate the black generation such that when there is high luminance there is less black. In the first of these approaches, the necessary amounts of black are achieved by layering the CMY dots. These methods have been in use for a long time.
There are several examples of lesser-used procedures. However, they encompass two main ideas. The first is to control the density of the dots of ink in a given region of the printed image. The second controls the color of the ink or inks used.
Examples of the first approach can be found in U.S. Pat. Nos. 4,713,746 (""746); 4,930,018 (""018); 5,386,305 (""305); and 5,648,801 (""801). In the ""746 patent, for example, the picture elements or pixels are restricted to be under 70% of the total area of the picture. This reduces the roughness of the image. A similar technique is used in the ""305 patent by classifying areas as intermediate and high density and controlling the amount of black in that area.
Other approaches, such as that found in the ""018 patent, the gray scale ink drop count and associated dye loading of the ink are tightly controlled to maximize the uniform distribution of the ink on the printed media.
Yet another approach, found in the ""801 patent, scans the incoming image several times. This produces several bit maps that are then handled as if they were part of a multicolor image. The image is then printed with a print head for each optical density bitmap, thereby controlling the densities of the print dots.
The second type of approach controls the ink concentration or density itself. Examples can be found in U.S. Pat. Nos. 4,860,026 (""026); 5,767,876 (""876); 5,142,374 (""374); 5,795,082 (""082); 5,734,800 (""800); and 5,805,178 (""178). For example, the ""026 patent uses two different concentrations of ink. Each pixel of the printed image is made up of several dots. The dots could be from the high concentration ink, the low concentration ink, or a combination of both. A similar technique is used in the ""876 patent.
In another approach found in the ""374 patent the generated data for dark ink is converted into a binary signal and used to reproduce the resolution. The generated data for the light ink is converted into a separate signal, used to reproduce gradient. These two signals and the control of the discharge of both inks is used to record the image. A similar method is used with regards to a dark ink looked up from a table in the ""082 patent.
Some techniques use what is referred to as six-color printing. The technique of the ""800 patent uses two inks of cyan, two of magenta, yellow and one black, often referred to in short-hand version as CCxe2x80x2MMxe2x80x2YK. One technique uses six-color printing with at least of the three inks having a fluorescent response. The use of the fluorescent response of the inks the image is smoothed out to avoid graininess.
In the ""178 patent, the range of pixel intensities are divided into three bands. Cyan and magenta are halftoned, with four levels per pixel. In the darkest band, the darker ink is always present and the lighter is halftoned. In the intermediate band is halftoned between the light ink and the darker ink. The light bank uses only halftoned light ink. However, it must be noted that only light inks are used for the colors of CMY. There are no examples in the art of using light ink for black.
Both types of solutions have problems. The methods that modulate the generation of black dots in the image regions use dark ink and cannot remove the noticeable black dots. The methods using two ink concentrations do not use light black. The methods that avoid the use of black, using a combination of C M and Y to generate black, but degrades the color gamut.
No solution exists that suggests the use of only one black ink, where that black ink is light ink.
One aspect of the invention is using light black ink to render color images in a printing system. The use of light black ink requires a different method to produce black generation values. The method uses two thresholds, where the value of the thresholds depends upon the density of the light black ink used. The first threshold is approximately where the cyan, magenta and yellow inks will begin to combine with black ink to produce black in the printed image. The second threshold is the point on a gray component density curve where the absolute value of black ink becomes a constant. The method determines a black generation value using these thresholds. Once the black generation value is determined, the other color values are adjusted to account for the black generation value.